Internet Engineering Task Force (IETF) Q. Wu, Ed.
Request for Comments: 6642 F. Xia
Category: Standards Track R. Even
ISSN: 2070-1721 Huawei
June 2012
RTP Control Protocol (RTCP) Extension for a Third-Party Loss Report
Abstract
In a large RTP session using the RTP Control Protocol (RTCP) feedback
mechanism defined in RFC 4585, a feedback target may experience
transient overload if some event causes a large number of receivers
to send feedback at once. This overload is usually avoided by
ensuring that feedback reports are forwarded to all receivers,
allowing them to avoid sending duplicate feedback reports. However,
there are cases where it is not recommended to forward feedback
reports, and this may allow feedback implosion. This memo discusses
these cases and defines a new RTCP Third-Party Loss Report that can
be used to inform receivers that the feedback target is aware of some
loss event, allowing them to suppress feedback. Associated Session
Description Protocol (SDP) signaling is also defined.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6642.
Wu, et al. Standards Track [Page 1]

RFC 6642 Third-Party Loss Report June 20121. Introduction
The RTP Control Protocol (RTCP) feedback messages [RFC4585] allow the
receivers in an RTP session to report events and ask for action from
the media source (or a delegated feedback target when using unicast
RTCP feedback with Source-Specific Multicast (SSM) [RFC5760]). There
are cases where multiple receivers may initiate the same, or an
equivalent, message towards the same media source or the same
feedback target. When the receiver count is large, this behavior may
cause transient overload of the media source, the network, or both.
This is known as a "feedback storm" or a "NACK storm".
One scenario that can cause such feedback storms involves video Fast
Update requests. A storm of these feedback messages can occur in
conversational multimedia scenarios like multipoint video switching
conference [RFC4587], where many receivers may simultaneously lose
synchronization with the video stream when the speaker is changed in
the middle of a session. Receivers that issue Fast Update requests
(i.e., Full Intra Request (FIR) described in RFC 5104 [RFC5104]), can
cause an implosion of FIR requests from receivers to the same media
source since these requests must currently be made blind, without
knowledge of requests made by other receivers.
RTCP feedback storms may cause short-term overload and, in extreme
cases, pose a possible risk of increasing network congestion on the
control channel (e.g., RTCP feedback), the data channel, or both. It
is therefore desirable to provide a way of suppressing unneeded
feedback. This document specifies a new Third-Party Loss Report for
this function. It supplements the existing use of RTCP NACK packets
and is also more precise in the uses where the network is active to
suppress feedback. It tells receivers explicitly that feedback for a
particular packet or frame loss is not needed and can provide an
early indication before the receiver reacts to the loss and invokes
its packet loss repair machinery. Section 3 provides some example
use cases of when to send the Third-Party Loss Report message.
2. Terminology2.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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RFC 6642 Third-Party Loss Report June 2012
As outlined in RFC 5760 [RFC5760], there are two Unicast Feedback
models that may be used for reporting: the Simple Feedback Model and
the Distribution Source Feedback Summary Model. In the Simple
Feedback Model, there's no need for the distribution source to create
the RTCP TPLRs; instead, RTCP NACKs are reflected by the distribution
source to the other receivers. However, in the Distribution Source
Feedback Summary Model, the distribution source will not redistribute
the NACK for some reason (e.g., to prevent revealing the identity or
existence of a system sending NACK) and may send an RTCP TPLR message
to the systems that were unable to receive the NACK and won't receive
the NACK via other means. The RTCP TPLR can be generated at the
distribution source when downstream loss is reported (e.g.,
downstream loss report is received), which indicates to the receivers
that they should not transmit feedback messages for the same loss
event for a certain time. Therefore, the distribution source in the
Distribution Source Feedback Summary Model can be reasonably certain
that it will help the situation (i.e., the distribution source is
unable receive the NACK) by sending this RTCP TPLR message to all the
relevant receivers impacted by the packet loss.
3.2. Unicast-Based Rapid Acquisition of Multicast Stream (RAMS) Use Case
The typical RAMS architecture [RFC6285] may have several Burst/
Retransmission Sources (BRSs) behind the multicast source placed at
the same level. These BRSs will receive the primary multicast RTP
stream from the media source and cache the most recent packets after
joining the multicast session. If packet loss happens at the
upstream of all the BRSs or the downstream of BRSs, one or all of the
BRSs may send an RTCP NACK or RTCP TPLR message to the distribution
source, where the SSRC in this RTCP NACK or RTCP TPLR message is the
BRS that is sending the message. The distribution source forwards/
reflects this message down on the primary SSM. The details on how
the distribution source deals with this message are specified in
[RETRANS-FOR-SSM].
3.3. RTP Transport Translator Use Case
A Transport Translator (Topo-Trn-Translator), as defined in RFC 5117
[RFC5117], is typically forwarding the RTP and RTCP traffic between
RTP clients, for example, converting from multicast to unicast for
domains that do not support multicast. The translator may suffer a
loss of important video packets. In this case, the translator may
forward an RTCP TPLR message received from upstream in the same way
it forwards other RTCP traffic. If the translator acting as the
monitor [MONARCH] is aware of packet loss, it may use the SSRC of the
monitor as the SSRC of the packet sender to create a NACK message and
send it to the receivers that are not aware of packet loss.
Wu, et al. Standards Track [Page 5]

RFC 6642 Third-Party Loss Report June 20123.4. Multipoint Control Unit (MCU) Use Case
When the speaker is changed in a voice-activated multipoint video
switching conference [RFC4587], an RTP mixer can be used to select
the available input streams and forward them to each participant. If
the MCU is doing a blind switch without waiting for a synchronization
point on the new stream, it can send a FIR to the new video source.
In this case, the MCU should send a FIR suppression message to the
new receivers. For example, when the RTP mixer starts to receive FIR
from some participants, it can suppress the remaining session
participants from sending FIR by sending out an RTCP TPLR message.
3.5. Mixer Use Case
A mixer, in accordance with RFC 5117 [RFC5117], aggregates multiple
RTP streams from other session participants and generates a new RTP
stream sent to the session participants. In some cases, the delivery
of video frames delivery may get damaged, for example, due to packet
loss or delayed delivery, between the media source and the mixer. In
such cases, the mixer needs to check if the packet loss will result
in PLI or FIR transmissions from most of the group by analyzing the
received video. If so, the mixer may initiate FIR or PLI towards the
media source on behalf of all the session participants and send out
an RTCP TPLR message to the session participants that may or are
expected to send a PLI or FIR. Alternatively, when the mixer starts
to receive FIR or PLI from some participants and would like to
suppress the remaining session participants from sending FIR or PLI,
it can just forward the FIR/PLI from one session participant to
others.
4. Protocol Overview
This document extends the RTCP feedback messages defined in the RTP/
AVPF [RFC4585] by defining an RTCP Third-Party Loss Report (TPLR)
message. The RTCP TPLR message can be used by the intermediaries to
inform the receiver that the sender of the RTCP TPLR has received
reports that the indicated packets were lost and ask the receiver not
to send feedback to it regarding these packets. Intermediaries are
variously referred to as distribution sources, Burst/Retransmission
Sources, MCUs, RTP translators, or RTP mixers, depending on the
precise use case described Section 3.
RTCP TPLR follows a similar message type format as RTCP NACK or Full
Intra Request Command. However, RTCP TPLR is defined as an
indication that the sender of the feedback has received reports that
the indicated packets were lost, while NACK [RFC4585] just indicates
that the sender of the NACK observed that these packets were lost.
The RTCP TPLR message is generated by an intermediary that may not
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RFC 6642 Third-Party Loss Report June 2012
have seen the actual packet loss. It is sent following the same
timing rule as sending NACK, defined in RFC 4585 [RFC4585]. The RTCP
TPLR message may be sent in a regular full compound RTCP packet or in
an early RTCP packet, as per the RTP/AVPF rules. Intermediaries in
the network that receive an RTCP TPLR MUST NOT send their own
additional Third-Party Loss Report messages for the same packet
sequence numbers. They SHOULD simply forward the RTCP TPLR message
received from upstream to the receiver(s). Additionally, they may
generate their own RTCP TPLR that reports a set of the losses they
see, which are different from ones reported in the RTCP TPLR they
received. The RTCP TPLR does not have retransmission request
[RFC4588] semantics.
When a receiver gets an RTCP TPLR message, it MUST follow the rules
for NACK suppression in RFC 4585 [RFC4585] and refrain from sending a
feedback request (e.g., NACK or FIR) for the missing packets reported
in the message, which is dealt with in the same way as receiving a
NACK.
To increase the robustness to the loss of a TPLR, the RTCP TPLR may
be retransmitted. If the additional TPLR arrives at the receiver,
the receiver SHOULD deal with the additional TPLR in the same way as
receiving the first TPLR for the same packet, and no additional
behavior for receiver is required.
A receiver may have sent a feedback message according to the RTP/AVPF
scheduling algorithm of RFC 4585 [RFC4585] before receiving an RTCP
TPLR message, but further feedback messages for those sequence
numbers SHOULD be suppressed after receiving the RTCP TPLR. Nodes
that do not understand the RTCP TPLR message will ignore it and might
therefore still send feedback according to the AVPF scheduling
algorithm of RFC 4585 [RFC4585]. The media source or intermediate
nodes cannot be certain that the use of an RTCP TPLR message actually
reduces the amount of feedback they receive.
5. Format of RTCP Feedback Messages
This document introduces two new RTCP feedback messages for Third-
Party Loss Report. Applications that are employing one or more loss-
repair methods MAY use the RTCP TPLR together with their existing
loss-repair methods either for every packet they expect to receive or
for an application-specific subset of the RTP packets in a session.
The following two sections each define an RTCP TPLR message. Both
messages are feedback messages as defined in Section 6.1 of RFC 4585
[RFC4585] and use the header format defined there. Each section
defines how to populate the PT, FMT, length, SSRC of packet sender,
SSRC of media source, and FCI fields in that header.
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RFC 6642 Third-Party Loss Report June 20125.1. Transport-Layer Feedback: Third-Party Loss Report (TPLR)
This TPLR message is identified by RTCP packet type values PT=RTPFB
and FMT=7.
Within the common packet header for feedback messages (as defined in
Section 6.1 of RFC 4585 [RFC4585]), the "SSRC of packet sender" field
indicates the source of the request, and the "SSRC of media source"
field denotes the media sender of the flow for which the indicated
losses are being suppressed.
The FCI field MUST contain one or more entries of Transport-Layer
Third-Party Loss Early Indication (TLLEI). Each entry applies to the
same media source identified by the SSRC contained in the "SSRC of
media source" field of the Feedback header. The length field in the
TLLEI feedback message MUST be set to N+2, where N is the number of
FCI entries.
The FCI field for TLLEI uses a similar message type format to that
defined in the Section 6.2.1 of RFC 4585 [RFC4585]. The format is
shown in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PID | BLP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Syntax of an FCI Entry in the TLLEI Feedback Message
Packet ID (PID): 16 bits
The PID field is used to specify a lost packet. The PID field
refers to the RTP sequence number of the lost packet.
bitmask of lost packets (BLP): 16 bits
The BLP allows for reporting losses of any of the 16 RTP packets
immediately following the RTP packet indicated by the PID. The
BLP's definition is identical to that given in Section 6.2.1 of
[RFC4585].
5.2. Payload-Specific Feedback: Third-Party Loss Report (TPLR)
This TPLR message is identified by RTCP packet type values PT=PSFB
and FMT=8, which are used to suppress FIR [RFC5104] and PLI
[RFC4585].
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RFC 6642 Third-Party Loss Report June 2012
Within the common packet header for feedback messages (as defined in
Section 6.1 of RFC 4585 [RFC4585]), the "SSRC of packet sender" field
indicates the source of the request, and the "SSRC of media source"
is not used and SHALL be set to 0. The SSRCs of the media senders to
which this message apply are in the corresponding FCI entries.
The FCI field for a Payload-Specific Third-Party Loss Early
Indication (PSLEI) consists one or more FCI entries. Each entry
applies to a different media source, identified by its SSRC, the
content of which is depicted in Figure 2. The length field in the
PSLEI feedback message MUST be set to N+2, where N is the number of
FCI entries.
The format is shown in Figure 2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Syntax of an FCI Entry in the PSLEI Feedback Message
Synchronization source (SSRC): 32 bits
The SSRC value of the media source that is already aware, or in
the process of being made aware, that some receiver lost
synchronization with the media stream and for which the PSLEI
receiver's own response to any such error is suppressed.
6. SDP Signaling
The Session Description Protocol (SDP) [RFC4566] attribute, rtcp-fb,
is defined in Section 4 of RFC 4585 [RFC4585] and may be used to
negotiate the capability to handle specific AVPF commands and
indications. The ABNF for rtcp-fb is described in Section 4.2 of RFC4585 [RFC4585]. In this section, we extend the rtcp-fb attribute to
include the commands and indications that are described for Third-
Party Loss Reports in the present document.
In the ABNF [RFC5234] for rtcp-fb-val defined in RFC 4585 [RFC4585],
the feedback type "nack", without parameters, indicates use of the
Generic NACK feedback format as defined in Section 6.2.1 of RFC 4585
[RFC4585]. In this document, we define two parameters that indicate
the third-party loss supported for use with "nack", namely:
o "tllei" denotes support of Transport-Layer Third-Party Loss Early
Indication.
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RFC 6642 Third-Party Loss Report June 2012
o "pslei" denotes support of Payload-Specific Third-Party Loss Early
Indication.
The ABNF for these two parameters for use with "nack" is defined here
(please refer to Section 4.2 of RFC4585 [RFC4585] for complete ABNF
syntax).
rtcp-fb-val =/ "nack" rtcp-fb-nack-param
rtcp-fb-nack-param = SP "tllei"
;Transport-Layer Third-Party
; Loss Early Indication
/ SP "pslei"
;Payload-Specific Third-Party
; Loss Early Indication
/ SP token [SP byte-string]
; for future commands/indications
token = <as defined in Section 9 of [RFC4566]>
byte-string = <as defined in Section 9 of [RFC4566]>
7. Security Considerations
The security considerations documented in [RFC4585] are also
applicable for the TPLR messages defined in this document.
More specifically, spoofed or maliciously created TPLR feedback
messages cause missing RTP packets to not be repaired in a timely
fashion and add risk of (undesired) feedback suppression at RTCP
receivers that accept such TPLR messages. Any packet loss detected
by a receiver that also receives a TPLR message for the same missing
packet(s) will negatively impact the application that relies on the
(timely) RTP retransmission capabilities.
A solution to prevent such attack with maliciously sent TPLR messages
is to apply an authentication and integrity protection framework for
the feedback messages. This can be accomplished using the RTP
profile that combines Secure RTP [RFC3711] and AVPF into SAVPF
[RFC5124].
Note that intermediaries that are not visible at the RTP layer that
wish to send the Third-Party Loss Reports on behalf of the media
source can only do so if they spoof the SSRC of the media source.
This is difficult if SRTP is in use. If the intermediary is visible
at the RTP layer, this is not an issue, provided the intermediary is
part of the security context for the session.
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